The long-term goal of this research is to understand the relationship between structure and function in the cell nucleus. We will concentrate on somatic and germ cells of Drosophila and on the giant nucleus or germinal vesicle (GV) of Xenopus oocytes. Chromosomes are the most prominent components of nuclei, but most nuclei also contain 3 other structures: nucleoli, Cajal bodies (CBs), and speckles (interchromatin granule clusters). The functions of the chromosomes and nucleoli are well understood, but the functions of CBs and speckles are much less clear. There is increasing evidence that parts of the RNA processing machinery of the nucleus are assembled in CBs before storage in the speckles and eventual transport to the chromosomes. We have proposed that CBs are also sites for assembly of parts of the transcription machinery itself. We will test this model by examining the newly-identified Drosophila CBs and by screening for Drosophila mutations that affect CB structure and function. We will look for specific assembly processes in CBs of Drosophila and Xenopus oocyte, and we will examine the movement of macromolecules between CBs, speckles, and the nucleoplasm. We will also study details of individual genes and their transcripts on the loops of the lampbrush chromosomes by fluorescent in situ hybridization and immunofluorescent staining. These techniques allow one to examine how individual transcription units are organized and how nascent transcripts are modified while they remain associated with the chromosome. We will use the same techniques to produce physical maps of the lampbrush chromosomes of Xenopus tropicalis, using genomic probes generated by the Xenopus tropicalis Genome Project. These studies will provide insight into the structure and function of the major nuclear organelles and how they interact during transcription and processing of nuclear RNA.